The invention relates to a method for removing scale and rust layers, especially on unalloyed steels prior to further treatment by phosphating (phosphatizing) and varnishing, using homogeneous organic pickling solutions based on low-boiling halogentated hydrocarbons as the basic component, containing aqueous phosphoric acid and optionally additionally sulfuric acid as the pickling agents, at least one low-boiling alcohol as the solubilizer, and at least one inhibitor for protection of the bare metal and/or for reduction of hydrogen generation. The pickling solutions of this invention can contain other activating, complex-forming, or reducing compounds.
It is known to free workpieces covered with scale and/or rust from the oxide or hydroxide layers by treatment in aqueous pickling baths in order to obtain completely bare surfaces [W. Rausch, E. G. Leuze Publishers, Saulgau (1974), "Die Phosphatierung von Metallen" (Phosphatizing of Metals), pp. 25-41]. The conventional pickling baths are usually inhibited, aqueous mineral acids, especially hydrochloric, sulfuric, or phosphoric acid [W. Rausch, E. G. Leuze Publishers, Saulgau (1974), "Phosphating of Metals", pp. 25-41]. The process is carried out in a concentration range from about 5 to 20% by weight and, depending on the pickling acid, in a temperature range from 20.degree. to 95.degree. C. A great variety of different classes of compounds can be utilized as inhibitors to prevent attack on the bare metal, such as, for example, aldehydes, amines, nitrites, sulfoxides, thioureas, thiosemicarbazides, and unsaturated alcohols. The pickling periods are normally between 10 and 30 minutes.
After pickling, the resultant salts must be carefully rinsed off with water. The surfaces of the workpieces must not dry off between pickling and phosphating, since drying results in a tendency toward formation of a thin film of rust on the metal surface. Surface rust interferes subsequent phosphating and leads to inadequate corrosion protection. Therefore, the process is usually carried out "wet-in-wet", to ensure satisfactory aqueous phosphating.
German Patent No. 1,236,301 describes homogeneous phosphating baths based on chlorinated hydrocarbons (CHC) with 0.05-7.5% H.sub.3 PO.sub.4 as the phosphating agent, 0.001-1% of a quinone as the hydrogen acceptor, and 1-10% of a lower alcohol as the solubilizer, obtaining phosphate coatings of about 1-2 g/m.sup.2.
According to U.S. Pat. No. 3,338,754, use is made of maximally anhydrous, homogeneous phosphating baths based on CHC with 0.05-6% H.sub.3 PO.sub.4 as the phosphating agent, 0.001 to 6% inhibitors, such as nitrobenzene, glacial acetic acid, and alkyl thioureas, 0.001-15% stabilizers, such as octyl phenol, diisobutylene, diethyl thiourea, tert-butyl pyrocatechol, and 1-35% solubilizer, such as lower primary alcohols, halogenated alcohols, acetic acid esters, dimethylformamide, and others, obtaining coating weights of about 300 mg/m.sup.2 to about 27 g/m.sup.2.
DOS No. 1,521,767 describes practically anhydrous, homogeneous phosphating baths based on CHC with 0.1-6% H.sub.3 PO.sub.4 as the phosphating agent, 0.01-6% of an organic nitro compound or 0.1-0.5% glacial acetic acid, and 1.5-25% of a lower alcohol as the solubilizer, thus obtaining coating weights of between about 270 mg/m.sup.2 to above 10 g/m.sup.2.
According to DAS No. 2,611,789 and DAS No. 2,611,790, homogeneous phosphating baths based on CHC contain up to 1% H.sub.3 PO.sub.4 as the phosphating agent, 1-6% H.sub.2 O, generally up to 25% of a lower alcohol as the solubilizer, and diverse additives of accelerators, stabilizers, and aprotic compounds. Coating weights of about 50 mg/m.sup.2 to 3.4 g/m.sup.2 can be produced therewith. The phosphoric acid concentration is not to be raised substantially above 1% since otherwise phosphate layers having a tacky feel are obtained.
In European application No. 34 842, aqueous, homogeneous phosphating baths are disclosed based on CH.sub.2 Cl.sub.2 with up to 2% H.sub.3 PO.sub.4, with 1-10% H.sub.2 O, with 10-50% of a lower alcohol, with the addition of small amounts of anionic or amphoteric tensides and accelerators, such as dinitrotoluene, urea or thiourea, producing phosphate layers of varying quality. The more recent phosphating methods based on organic solvents as disclosed, for example, in DAS No. 2,611,789 and DAS No. 2,611,790, EP-A No.34 842, require workpieces that are rust- and scale-free, as well as dry. This prerequisite is obtained in case of greased workpieces without other surface contaminants by simple degreasing, for example with a solvent, in a dipping and/or vapor treatment process.
Under practical conditions, the prerequisite of freedom from rust and scale of the workpieces is frequently unattainable, in spite of corrosion-protective greasing. In these cases, it is necessary not only to degrease the workpiece, but also to remove rust and scale in a suitable way.
Aqueous pickling is unsuitable in such instances, since during the subsequent solvent-based phosphating it is impossible to work "wet-in-wet", and thus surface rust formation and reduction in corrosion protection are unavoidable.
Pickling systems other than aqueous, though, have not been known at all heretofore.
A need therefore exists for a method of pickling and an organic pickling solution to avoid the disadvantages of aqueous pickling systems and to be compatible with organic solvent-based phosphating systems.